Many systems employ power converter circuits. These circuits receive electrical power in one form and convert it to another form, for example, to a form that is usable by electrical equipment employed within the particular system.
One type of power converter circuit is referred to as a switching type power converter circuit or simply a switching power supply. Switching type power converter circuits make use of switches, as well as capacitors, inductors and/or transformers, in order to convert the electrical power from one form to another. These switches have an on state and an off state. The on state is sometimes referred to as the closed state or the conducting state. The off state is sometimes referred to as the open state or the non-conducting state.
As with many power converter circuits, a switching type power converter circuit is often expected to operate with a particular level of efficiency and to provide a particular level of regulation over line and load changes.
The efficiency of a switching type converter circuit depends in part on the amount of power that is dissipated across the switches. The power loss across the switches is equal to the product of the voltage across the switch and the current through the switch. In this regard, the losses during the transitions from the on state to the off state, and vice versa, are often the main design concern. (When the switch is in the on state, the voltage across the switch is ideally zero. When the switch is in the off state, the current through the switch is zero.) Losses can occur during the transition from the on state to the off state, and vice versa, if there is a non-zero voltage across the switch and non-zero current through the switch. Such losses are proportional to the product of the power lost per transition and the switching frequency. Therefore, to reduce the losses across a switch, a zero-current condition is desired while the switch transitions from the on state to the off state, and a zero-voltage condition is desired while the switch transitions from the off state to the on state.
Several techniques have been introduced, which accomplish zero-voltage switching inherently at constant switching frequency. One of these techniques requires a full-bridge switching arrangement with four primary switches in which the regulation is accomplished by shift phase modulation. This technique has several drawbacks including the limited availability of phase-modulated integrated control circuits and the large number of parts, which include four primary switches, at least two secondary switches and at least two large magnetic circuit elements. The technique suffers from an inability to accomplish zero-voltage switching at light loads without additional circuit elements and additional complexity.
Another circuit to address this purpose is based on the single-ended forward converter that accomplishes zero-voltage switching by addition of an extra primary side switch and capacitor. Disadvantages of this converter include in additional voltage stress on the primary switching elements required to reset the transformer core. The parts required are two large magnetic circuit elements, the transformer and the filter inductor, two primary switches, a large primary capacitor, and two secondary switching elements.
There is one example of prior art that accomplishes a zero-voltage switching converter, which has a single magnetic circuit element, accomplishing both magnetic energy storage and isolation. This converter relies on high AC magnetizing fields in order to accomplish zero-voltage switching, requiring that the magnetizing field and the magnetizing current change sign during each cycle. However, these increased losses impose a limit on the level of power density and efficiency that can be obtained with this approach.
Notwithstanding the performance level of current switching type power converter circuits, further improvements are sought.